The sole plates of screeds on asphalt paving machines must be preheated to about 175.degree. F. to 200.degree. F. before paving commences to keep the hot asphalt from congealing on the sole face of the screeds. Prior to my inventions identified above, the preheating has been accomplished by oil or propane burners mounted on the moldboard and directly heating the top surface of the screeds. When using such burners, particular care must be taken to avoid overheating since this can result in permanent warping of the screed.
My prior invention provided an improved preheating system applicable to both primary and extension screeds which eliminated the use of oil or propane burners by utilizing the heat created by dropping the pressure of high pressure oil at a flow restrictor. In the case of extender screeds, a reservoir was initially mounted in direct contact with the top surface of each screed, and oil from the reservoir was pressurized by a pump and circulated through a flow restrictor to create a large pressure drop. This pressure drop is adjusted to result in an output oil temperature of about 275.degree. F. which normally is sufficient to establish the desired screed temperature of about 200.degree. F.
In the case of the primary screed, instead of providing a reservoir on the top of the screed, my previous invention provided flat-sided heat exchange tubes attached directly to the upper surface of the screed through which the heated oil is circulated. This arrangement reduces the amount of oil required in the system.
In the preferred operation of my previous invention directed particularly at heating a primary screed for an asphalt paver having a primary floating screed equipped with screed extensions and towed by a tractor, a hydraulic pump on the tractor supplies pressurized oil to a control unit on the primary screed from which the oil is selectively distributed to drive hydraulic motors in the heating and vibratory systems for the screeds. The hydraulic motor in the heating system drives a high pressure pump whose output is circulated in a closed independent system through a flow restriction causing the circulating oil to be heated responsive to the resulting pressure drop. Heated oil is circulated through heat exchange tubes mounted on the sole plate of the primary screed and returns to a small reservoir tank from which the high pressure pump takes its suction. Heated oil is also circulated through head exchange tubes on the sole plates of the extender screeds or through shallow tanks on the sole plates. One or more temperature sensors are is provided in the heating system to detect overheating of the components and cause most of the oil supplied to the hydraulic motor or motors in the heating system to be cut off in the control manifold when an overheating condition is sensed. After being utilized to heat the screeds, the pressurized oil from the tractor is diverted at the control manifold to hydraulic motors in the vibratory system which drive eccentric carrying shafts mounted on the screed units.